aww bless you, deffo have sex in the light men tend to rely on visuals to be turned on. As everyone has said its probs nerves and tiredness.
If it never gets better in time i would perhaps suggest goin to your gp?

don't worry dude, just ads to the problem. i fail to perform time to time, no excuses. i was fine for ages then two nights on the flop and all my confidence went to pot. i took viagra( not a whole one) and i just felt guilty. if your gf loves you she will be patient, she migh get frustrated but it happens to the best of us.

Nerves and tiredness can contribute to this! Just dont get wound up about it...because im sure that makes it more difficult to try then. Just be relaxed, keep up the foreplay before hand and if you do start to ya know...dont stress about it, just get some more foreplay going on hopefully once your fully comfortable with each other sexually through foreplay and stuff the nerves should go away

me and boyfriend never have had this problem but i usually buy sexy underware, get randy in public, have sex all over the house, do strip teases, always do dirrerent postions, just try being sluts with each other, that should help, and just let it happen, dont plan it, the best sex is when your laying in bed going to sleep and then you just feel a kiss on your neck, and things just flow.